null pointer questions

null pointer questions

[Ken Ashcraft]

Two questions:
1. If I pass size 0 to kmalloc, what does it return?

2. What happens if I pass a null pointer as the destination parameter
to copy_from_user?  Does copy_from_user handle it safely or will the
kernel seg fault?

Thanks for your answers,
Ken Ashcraft

Re: null pointer questions

[Roman Zippel]

Hi,

On Thu, 1 Nov 2001, Ken Ashcraft wrote:

> 1. If I pass size 0 to kmalloc, what does it return?

AFAIK size is always rounded up, so you get the smallest possible
allocation unit.

> 2. What happens if I pass a null pointer as the destination parameter
> to copy_from_user?  Does copy_from_user handle it safely or will the
> kernel seg fault?

The kernel won't crash, but it might fail (depending on whether 0 is a
valid user space address or not).

bye, Roman

Re: null pointer questions

[lkml user]

On Fri, 2 Nov 2001, Roman Zippel wrote:

> Hi,
> 
> On Thu, 1 Nov 2001, Ken Ashcraft wrote:
> 
> > 1. If I pass size 0 to kmalloc, what does it return?
> 
> AFAIK size is always rounded up, so you get the smallest possible
> allocation unit.
> 
> > 2. What happens if I pass a null pointer as the destination parameter
> > to copy_from_user?  Does copy_from_user handle it safely or will the
> > kernel seg fault?
> 
> The kernel won't crash, but it might fail (depending on whether 0 is a
> valid user space address or not).
> 
> bye, Roman
> 
> 
> -
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Re: null pointer questions

[Ken Ashcraft]

> > 2. What happens if I pass a null pointer as the destination parameter
> > to copy_from_user?  Does copy_from_user handle it safely or will the
> > kernel seg fault?
>
> The kernel won't crash, but it might fail (depending on whether 0 is a
> valid user space address or not).

Why does it matter if 0 is a valid user space or not?  If I make the call

copy_from_user(0, user_ptr, 4);

the null pointer is the kernel address, not the user address.  Can you
clarify please?

Thanks
Ken

Re: null pointer questions

[Roman Zippel]

Hi,

Ken Ashcraft wrote:

> Why does it matter if 0 is a valid user space or not?  If I make the call
> 
> copy_from_user(0, user_ptr, 4);
> 
> the null pointer is the kernel address, not the user address.  Can you
> clarify please?

Sorry, I misunderstood you. The kernel address has to be a valid address
of course, otherwise the behavior is undefined.

bye, Roman

Re: null pointer questions

[Brian Gerst]

Ken Ashcraft wrote:
> 
> > > 2. What happens if I pass a null pointer as the destination parameter
> > > to copy_from_user?  Does copy_from_user handle it safely or will the
> > > kernel seg fault?
> >
> > The kernel won't crash, but it might fail (depending on whether 0 is a
> > valid user space address or not).
> 
> Why does it matter if 0 is a valid user space or not?  If I make the call
> 
> copy_from_user(0, user_ptr, 4);
> 
> the null pointer is the kernel address, not the user address.  Can you
> clarify please?

copy_from_user uses the string move instruction on the x86, so the
exception code would assume the source faulted not the dest.  It would
return -EFAULT instead of causing an oops.

-- 

						Brian Gerst

Re: null pointer questions

[Brian Gerst]

Brian Gerst wrote:
> 
> Ken Ashcraft wrote:
> >
> > > > 2. What happens if I pass a null pointer as the destination parameter
> > > > to copy_from_user?  Does copy_from_user handle it safely or will the
> > > > kernel seg fault?
> > >
> > > The kernel won't crash, but it might fail (depending on whether 0 is a
> > > valid user space address or not).
> >
> > Why does it matter if 0 is a valid user space or not?  If I make the call
> >
> > copy_from_user(0, user_ptr, 4);
> >
> > the null pointer is the kernel address, not the user address.  Can you
> > clarify please?
> 
> copy_from_user uses the string move instruction on the x86, so the
> exception code would assume the source faulted not the dest.  It would
> return -EFAULT instead of causing an oops.

Err, would return non-zero instead of -EFAULT.

-- 

						Brian Gerst